The present invention relates to a control system for an internal combustion engine, and more particularly to a control system having a control device such as an exhaust gas recirculation mechanism or an evaporative fuel processing system, that affects the air-fuel ratio of an air-fuel mixture to be supplied to the internal combustion engine. In this specification, the control device that affects the air-fuel ratio is referred to as “A/F affecting control device”.
An exhaust gas recirculation mechanism for recirculating exhaust gases from an exhaust system of an internal combustion engine to an intake system thereof, is widely used for improving exhaust characteristics of the engine. An evaporative fuel processing system in which a canister containing an adsorbent stores evaporative fuel generated in a fuel tank to supply the evaporative fuel to the intake system during operation of the engine, is also widely used for preventing emission of the evaporative fuel to the atmosphere. A control device such as the exhaust gas recirculation mechanism or the evaporative fuel processing system, affects the air-fuel ratio of an air-fuel mixture to be supplied to the engine. Accordingly, in some instances an abnormality such as clog or a leak in an exhaust gas recirculation passage or an evaporative fuel passage, may have an adverse effect on the air-fuel ratio control.
The recent tightening of emission regulations (harmful gas emission) has highlighted that the amount of exhaust gases to be recirculated to the intake system or the amount of evaporative fuel to be supplied to the intake system changes due to a clog or similar abnormality in the exhaust gas recirculation passage or the evaporative fuel passage. Such changes have an adverse effect on the exhaust characteristics of the engine.
One example of a method for determining an abnormality such as a clog in an exhaust gas recirculation passage is described in Japanese Patent Laid-open No. Hei 7-180615. According to this method for determining an abnormality, an intake pressure at an open position of an exhaust gas recirculation valve and an intake pressure in a closed position of the exhaust gas recirculation valve are measured during engine operation when the fuel supply to the engine is cut off. If the difference between these intake pressures is smaller than a predetermined value, the exhaust gas recirculation mechanism is judged as abnormal because there is a clog or a leak in the exhaust gas recirculation passage.
Further, a method of correcting an air-fuel ratio with an air-fuel ratio control system is known from Japanese Patent No. 2576481. The air-fuel ratio control system includes an air-fuel ratio sensor provided in an exhaust system of an internal combustion engine. An air-fuel ratio correction coefficient is calculated according to an output from the air-fuel ratio sensor to control the air-fuel ratio of an air-fuel mixture to be supplied to the engine so that it becomes equal to a target air-fuel ratio. According to this method, a learning correction value is calculated by using the air-fuel ratio correction coefficient during execution of exhaust gas recirculation, and a deviation of the air-fuel ratio due to aging is corrected by using the learning correction value.
The method for determining an abnormality as described in Japanese Patent Laid-open No. 7-180615, mentioned above, is executed in the fuel-cut operation of the engine, which is a very narrow and limited operating condition. Accordingly, there is a limit to increasing the frequency of the determination and it is difficult to do so.
On the other hand, in the above method described in Japanese Patent No. 2576481, the learning correction value is calculated by a simple averaging of air-fuel ratio correction coefficients regardless of changes in the engine operating condition and in the exhaust gas recirculation amount. Accordingly, it is difficult to obtain an accurate learning correction value that is applicable over a wide range of engine operating conditions.